1. Field of the Invention
The present invention relates to an optical measuring system based on Raman scattering. It is a stand-off system for detecting and identifying chemical threat substances, which systems can be optimized for different scenarios.
2. Description of Related Art
During the last decades several terrorist attacks have been carried out, almost all with different types of IED:s (Improvised Explosive Device). Some have been placed in boxes, other in bottles or vehicles. This makes it very difficult to know what to look for when searching for IED:s. One common thing for all IED:s are, however, that they contain some kind of explosive material. The explosive itself is rarely visible from the outside; traces of the explosive can nevertheless almost always be found outside the IED in form of particles or molecules in the air surrounding it.
It is not trivial to detect and identify these traces. Developments during the last years of smaller pulsed lasers and high sensitive detectors have shown, however, stand-off optical methods such as Raman, fluorescence and infrared spectroscopy to be promising and usable techniques. Unfortunately no existing method can be used for all cases due to the different forms of the explosive material to be detected and identified, such as vapour, particles, liquid and bulk. It has so far been necessary to use a specialized instrument for each case.
One of the main limitations of existing detectors is that they can only record two dimensions (2-d), at the same time. As an example, one dimension can be spatial and one dimension spectral. Such equipment can be used to detect explosives in a number of cases where two spatial dimensions are not important. They have a problem, however, in finding small particles, where two spatial dimensions are vital in order to be able to distinguish between the spectrum from a particle and the background surface.
For a “non-moving” object can done by either a) scanning the spatial dimensions and record the full spectrum from each point on the surface or b) use a tunable filter and thereby collect two spatial dimensions and scan the spectrum. Both of these techniques have their own area of use, but both of these techniques are time consuming. A demand on a new measuring system is that one and the same system should able to cover every different measuring scenario, and in order to be able to do so it must have the possibility to record the two spatial dimensions as well as the spectral dimension at the same time
Below follows examples of scenarios that demands different measuring equipment.
In a first scenario, the Police receive an alarm of a suspicious bomb and arrive on the scene. Initially, high precaution must be taken and the examination must start from a long distance. This is more or less only possible if there are large quantities of the unknown substance visible, e.g. if it is in a transparent bottle or container. The air above the substance could also be examined by resonant Raman spectroscopy (RRS) to detect explosive vapour from a bomb. In both of these cases point detection can be used.
If nothing is found, the Police may move closer to the object to be able to find traces of explosives outside e.g. in fingerprints. In this case, very sensitive techniques such as 2-d spectral imaging must be used. If nothing is detected still and the container is semi-opaque, e.g. of plastic, it is possible to use spatial offset Raman spectroscopy (SORS) to measure through the container.
In a second scenario, a suspicious car outside a building is reported. The Police that arrive on the scene may start by using imaging Raman spectroscopy from outside the car in search for explosive particles e.g. at the door, seat belts, steering wheel or the trunk, where traces are most likely to exist. If it is possible to measure into the car it is also possible to examine bulk substances using point detection. Any semi-opaque container might be examined by using SORS.
In a third scenario, a military troop has found a possible roadside bomb. First the air above the possible bomb may be scanned using point detection. If nothing is found, the object itself and the ground around it can be scanned using imaging techniques to find explosive particles. Again, if the container is made of a semi-opaque material, the object can be scanned using SORS to identify the substance inside.
As has been exemplified, there are a number of known, different types equipment using Raman spectroscopy that are each suitable for certain specific measuring conditions. There exists, however, no measuring equipment that can be used for many, and absolutely not for all, measuring modes, and especially not one that is able to be reconfigured rapidly for different measuring modes.
One way of categorizing different measuring modes are:
1. 0-dimensional; point measurement
                a. Bulk detection; measurement at larger quantities and at larger maximum distances <500 m        b. Gas phase detection; measurements on the air above a suspicions object, Resonant Raman Spectroscopy (RRS)        c. Detection of smaller objects at well-defined locations2. 1-dimensional; line measurement        a. Bottle detection; measurement through semi-opaque containers, Spatial Offset Raman Spectroscopy (SORS)        b. Surface detection; measurement on flat surfaces with object in well-defined locations, scanning        c. Particle detection; trace measurement from e.g. fingerprints, scanning3. 2-dimensional; surface measurement        a. Particle detection; trace measurement from e.g. fingerprints, imaging        b. Bottle detection; measurement through semi-opaque containers, imaging or Spatial Offset Raman Spectroscopy (SORS)        